Phenolic resin moldings are excellent in mechanical properties and hence have widely been employed for long either independently or in the form of a blend with another resin, such as an epoxy resin. However, the phenolic resin moldings have drawbacks in that the light and alkali resistances thereof are relatively low, that they are liable to absorb moisture or an alcohol and thereby suffering in some problems, such as change in the dimension and change of electrical resistance, and that the thermal resistance properties, especially the oxidation resistance at high temperature, thereof is poor.
In order to overcome these drawbacks, various modifications of the phenolic resin have been studied. For example, a variety of modified phenolic resins have been proposed, which have improved the resistances thereof to the deterioration and oxidation due to light, chemicals, etc. by virtue of the modification using a fat and oil, a rosin or a neutral aromatic compound.
Among the various proposals, for example, Japanese Patent Laid-open Publication No. 61(1986)-235413 discloses obtaining a phenolic resin of excellent heat resistance by using particular reactants of a phenol-modified aromatic hydrocarbon resin. However, the phenolic resin obtained by this method is disadvantageous in that, in the manufacturing of a molding by the use thereof, the phenolic resin cannot be cured unless being maintained at a high temperature for a prolonged period of time.
Japanese Patent Laid-open Publication No. 2(1990)-274714 discloses that a modified phenolic resin useful as a molding material, having such excellent heat and oxidation resistances and mechanical strength as cannot be expected from the conventional phenolic resin, which modified phenolic resin is obtained by employing as a modifier a petroleum heavy oil or pitch, which materials are commercially available at a low cost and using specific reaction conditions.
Further, Japanese Patent Laid-open Publication Nos. 4(1992)-145116 and 6(1994)-228257 disclose that, in the production of such a modified phenolic resin, a modified phenolic resin which would not corrode any metal members brought into contact with the resin can be provided by subjecting a crude modified phenolic resin obtained by a polycondensation of starting compounds to a neutralization treatment, a water washing treatment and/or an extraction treatment to thereby neutralize or remove any acids remaining in the crude modified phenolic resin.
However, the above modified phenolic resin has a drawback in that the melt viscosity thereof is so high that the modified phenolic resin is not suitable for rapid mass production of a molding with complex configuration. Moreover, there has been a demand for further enhancement of the heat resistance and mechanical properties, such as dimensional stability and strength, of a molding produced from the modified phenolic resin in combination with an epoxy resin.
In these circumstances, we, the present inventors have been proposed a process for producing a modified phenolic resin, in which a modified phenolic resin obtained by a polycondensation of a petroleum heavy oil or pitch, a phenol and a formaldehyde polymer, is reacted with a phenol in the presence of an acid catalyst so as to attain a reduction of the molecular weight with the result that a modified phenolic resin having a low melt viscosity and improved in the reactivity with an epoxy resin can be obtained (see Japanese Patent Laid-open Publication Nos. 7(1995)-252339 and 9(1997)-216927).
The modified phenolic resin thus obtained exhibits a high reactivity with an epoxy resin and has a low melt viscosity to thereby ensure an excellent moldability. Thus, in combination with the epoxy resin, the modified phenolic resin can provide a molding material which is excellent in not only heat resistance and moldability but also mechanical properties such as dimensional stability.
However, the known modified phenolic resins obtained by these processes, because being solid at about room temperature, must be heated so as to attain softening or melting thereof and inserted under pressure in a molding device in carrying out molding, for example, transfer molding.
On the other hand, in the use in molding raw materials, especially raw material for semiconductor seals, various package configurations are being proposed in conformity with the recent-year advance of the reduction of package size and weight. Extensive investigations are being conducted on liquid sealers that would not need a large-scale molding device in order to cope with the requirement for use of a large variety of packages in small amounts.
It is customary to employ acid anhydride and amine curing agents for the curing of epoxy resins for use in liquid sealers. These curing agents, however, have been unsatisfactory from the viewpoint of moistureproof reliability required for the above packages of reduced size and weight.
The use of phenolic resins, especially the modified phenolic resins above mentioned, as the curing agents would improve the performance such as moistureproof reliability. However, the phenolic resins are solid at about room temperature, so that it has been difficult to obtain liquid sealers exhibiting satisfied flowability on the use thereof.
The inventors have conducted various investigations and studies with a view toward developing a process capable of producing a modified phenolic resin of low softening point and low melt viscosity while maintaining the excellent performance of conventional modified phenolic resins. As a result, it has been found that a modified phenolic resin exhibiting flowability even at about room temperature and having low melt viscosity can be produced by polycondensation of a heavy oil or pitch, a formaldehyde compound and a specific phenol having unsaturated hydrocarbon group(s). The present invention has been completed on the basis of this finding.
The present invention has been made with a view toward solving the above problems of the prior art. Accordingly, the object of the present invention is to provide a modified phenolic resin which, as compared with the conventional modified phenolic resin produced from a heavy oil or pitch as a feedstock, has an extremely low softening point, therefore being flowable even at about room temperature (25.degree. C.), and has also a low melt viscosity, thereby ensuring very excellent moldability, and which modified phenolic resin, in combination with an epoxy resin, can be formed into a molded item being excellent in not only heat resistance, moisture resistance, corrosion resistance and adhesive property but also mechanical properties such as dimensional stability and strength.